Batch digestion of wastewood

ABSTRACT

A PROCESS FOR PRODUCING PULP FROM WASTEWOOD IN A BATCH DIGESTING OPERATION WHICH COMPRISES, (A) PRIOR TO DIGESTING, THOROUGHLY MIXING WASTEWOOD WITH COOKING LIQUOR AT A LIQUOR TO WASTEWOOD RATIO OF BETWEEN 3:1 AND 7:1, (B) INTRODUCING THE MIXTURE INTO A BATHC DIGESTER AND HEATING THE MIXTURE AT A TEMPERATURE FROM ABOUT 280*F. TO 320*F. AND A PRESSURE OF FROM 30 TO 70 P.S.I.G. FOR FROM ABOUT 15 TO 50 MINUTES, AND (C) DIGESTING AT A TEMPERATURE FROM 330*F. TO 360*C. AND PRESSURE OF 90 TO 120 P.S.I.G. THE TOTAL COOLING TIME IS 90 TO 180 MINUTES, PRODUCED FROM WASTEWOOD BY THIS PROCESS IS CONPULP PRODUCED FROM WASTEWOOD BY THIS PROCESS IS CONSISTENLY CLEAN AND UNIFORM.

July 24, 1973 J MOLE ET AL BATCH DIGESTION OF WASTEWOOD Filed June 13, 1972 WASTEWOOD CHEMICAL PROPORTIONING CONTROLLER WHITE LIQUOR SUPPLY BLACK LIQUOR TEMP. U RECORDER MIXING CONVEYOR B A T (I H D I G E S T E R T0 BLOW TANK STEAM HGT 6 m m X M m w W W D D 0 0 0 0 w w E E T T M M W W T T E E W W X X x x m w w w o 5E 20 2.33m 50 o 20 F m G nm MW RN W D "O mm m m F NW N MMW WE OKOO WJ [A 7 AA nu TILII S W W W M 0 T z cnwTA c. .BR M DBE GEM MFOTT UU I O II o MM 8 W :4... Q

United States Patent Int. Cl. D21c 7/12 US. Cl. 162-61 4 Claims ABSTRACT OF THE DISCLOSURE A process for producing pulp from wastewood in a batch digesting operation which comprises, (a) prior to digesting, thoroughly mixing wastewood with cooking liquor at a liquor to wastewood ratio of between 3:1 and 7:1, (b) introducing the mixture into a batch digester and heating the mixture at a temperature from about 280 F. to 320 F. and a pressure of from 30 to 70 p.s.i.g. for from about 15 to 50 minutes, and (c) digesting at a temperature from 330 F. to 360 F. and pressure of 90 to 120 p.s.i.g. The total cooking time is 90 to 180 minutes, preferably 105 to 120 minutes. The chemical pulp produced from wastewood by this process is consistenly clean and uniform.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a process for consistently producing clean, uniform pulp from wastewood in a batch digester using the Kraft pulping process. More specifically, this invention relates primarily to thoroughly mixing wastewood with cooking liquor prior to digestion in a batch-type digester to render it suitably clean as prime Kraft pulp.

(2) The prior art Tremendous quantities of wastewood residuals are being discarded or burned each year by the lumber and woodworking industries. These represent a large potential source of cellulose fiber for the pulp and paper industry. Past attempts to exploit this resource have met with many failures. The most successful processes have involved specialized continuous digesting equipment. These are very expensive and often lack flexibility as to pulp quality. Since most pulp mills use batch type stationary digesters, the successful use of such equipment would enable much more widespread usage of this wastewood resource. When wastewood has been cooked in batch digesters, however, poor pulp quality has been a frequent result. Occasional pockets of wastewood within the digester escape proper cooking and contaminate the resulting pulp. This uncooked wastewood is extremely diflicult to remove by fiberizing or refining equipment or by screens and centrifugal cleaners. The dirty pulp is, therefore, of little commercial value.

Another problem with cooking wastewood has been the inability to accurately measure the actual amount of wood substances charged per cooking batch. If, for example, it comes from kiln-dried wood, the moisture content may be as low as 10%. On the other hand, green sawdust may contain about 55% moisture and heavy rains during transit to the mill may increase the moisture content to 60 to 65%. These variations make charging digesters diflicult both from the standpoint of adding the proper amount of alkali, and in maintaining the proper liquor-to-wastewood ratio.

A third problem with cooking wastewood has been the inherent variability of the fibrous material. For example, sometimes the wastewood is in the form of fine particle sawdust; whereas at other times the wastewood is in the form of shavings and still at other times a mixture.

In charging batch digesters, the individual wastewood particles pack tightly together. They thus present a myriad of relatively small tortuous pathways through which cooking liquor can penetrate the mass and through which the displaced air must also escape. It is believed that the penetrating liquor frequently by-passes a portion of wastewood leaving it still unwetted and immersed in air. Phenomena of this kind have been shown to occur in the process of displacing petroleum from an oily sand or sandstone by water. This is known as the Jamin effect. When wastewood has been by-passed by the liquor it does not take part in the normal pulping reactions. It ends up instead as essentially uncooked wood dirt in the final pulp.

Entrapped air in the wastewood mass causes problems because it effectively prevents the penetration of cooking liquor to the wastewood particles it contains. The air also thermally insulates these enclosed particles so they do not increase in temperature as rapidly as do particles completely immersed in the liquid phase. Both these actions result in uncooked or undercooked wastewood particles and consequently dirty pulp. These trapped air pockets are sometimes dislodged during the cooking process. Upon heating, the air expands and the increased buoyancy often enables it to break through the wastewood structure above it and thus escape. Unfortunately, by that time in the process the liquor that displaces the air is deficient in active cooking chemical. The originally unpenetrated wastewood is thus only partially cooked and still ends up as unfiberized Wood dirt.

It should be pointed out that other methods of cooking wastewood have been shown to give clean cooks a large part of the time. However, the occurrence of even one occasional dirty cook is so disastrous that these processes are uneconomic. This is true despite the relatively low price of the raw material. As a result, conventional cooking of wastewood in batch digesters has been largely abandoned as pulps from dirty cooks are particularly difficult to clean to a useful degree.

Furthermore, the ability to pulp successfully wastewood eliminates a substantial environmental problem. First that portion of the tree processed to wastewood has heretofore been thrown away or burned, usually in Wigwam burners thereby polluting the air. Secondly, because consistent results are now obtained there are no bad cooks which have been disposed of occassionally through a waste stream.

The present invention includes two steps to prevent the consequence of the Jamin effect. The pre-mixing of the cooking liquor and the wastewood insures that each wastewood particle is wet with full strength cooking chemicals. It is therefore, less dependent on subsequent liquor circulation and movement Within the digester. The twostage cooking step also encourages the buoyancy breakup of air pockets at relatively low temperatures. The wastewood mass is thus enabled to reach uniform temperature during the second and active cooking stage. As a result, the dirty pulp problem is essentially eliminated.

It is therefore, a general object of this invention to provide a process for producing consistently a clean, uni form pulp from both kiln-dried and green wastewood as well as mixtures thereof. Another object of this invention is to provide a process for producing a clean, uniform pulp from wastewood in a conventional batch-type digester. Still another object of this invention is to provide a process for thoroughly diffusing cooking liquor throughout a dense mass of wastewood material and subsequently digesting. A still further object of this invention is to provide a process having the moisture controls necessary for providing the proper amount of cooking liquor.

Other objects, features and advanta es of this invention will be readily apparent from the following detailed description of the process of this invention with reference to the accompanying drawing in which;

FIG. 1 shows a schematic diagram outlining the process of this invention, and

FIG. 2 illustrates the relationship of rejects as a function of the liquor to wastewood ratio with and without mixing of the wastewood with the liquor.

SUMMARY OF THE INVENTION It has been found that consistently clean, uniform pulp may be obtained in a batch digester by first thoroughly mixing cooking liquor and either kiln-dried or green wastewood prior to charging of the digester. The mixing may be done using any mechanical equipment that will accomplish thorough wetting of the wastewood material, for example, a screw conveyor, Next, introducing the mixture to a batch digester and heating the mixture at a temperature from about 280 F. to 320 F. and a pressure of from 30 to 70 p.s.i.g. for from about 20 to 50 minutes, and digesting at a temperature from 330 F. to 360 F. and pressure to 90 to 120 p.s.i.g. The total cooking time is from 90 to 180 minutes, preferably from 105 to 120 minutes. The chemical pulp produced from wastewood by this process is consistently clean and uniform.

DETAILED DESCRIPTION OF THE INVENTION As stated above, this invention relates to cooking wastewood. By the term wastewood, it is meant those lignocellulosic materials that are the by-product of lumber milling operations, known as sawdust and shavings and which are substantially free of (bark. The term is also meant to include other wastewood materials of typical lignocellulosic material smaller than wood chips. Wastewood is also intended to encompass both kiln-dried wastewood having a low moisture content, i.e., below about 15 green wastewood having a high moisture content, i.e., 30'-6 5% moisture by weight, and mixtures thereof. However, it is preferable that the wastewood have a moisture content above, say 20% by weight prior to premixing.

Referring to FIG. 1, wastewood is conveyed from storage through conveyor means 9 to a Weightometer and from the Weightometer past a moisture gauge to mixing screw conveyor or other appropriate mixing means, wherein Kraft white cooking liquor is introduced. Operation of conveyor 9 is automated and will shut off at a preset O.D. wood weight when signaled by the Chemical Proportioning Controller. As the liquor to wastewood ratio is important to the success of the process, it is necessary to know the moisture content of the wastewood so that the proper amount of liquor may be mixed. The amount of actual cooking liquor per unit of dry wood substance is selected and entered in the Chemical Proportioning Controller.

The moisture content of the wastewood may be determined by a number of means; however, an infra-red absorption gauge has been shown to be eflective since it instantaneously measures the moisture content of the wastewood. A signal from the moisture gauge is combined in the Chemical Proportioning Controller with a signal from the Weightometer. From the quantities, the weight per unit time of oven-dried wood being charged is calculated and the quantity of cooking liquor to be supplied at the mixing conveyor is calculated and controlled either manually by the operator, or automatically if a more sophisticated system is available, through valve 10. It is important that the liquor-to-wastewood ratio be in the proper balance if a clean pulp is to be obtained. The white liquor and wastewood are mixed in the mixing conveyor by any suitable mixing means so long as the wastewood particles are each wetted with the white liquor. It has been found that mixing screw conveyors are especially satisfactory for accomplishing thorough mixing. All or a portion, depending upon the moisture content of the wastewood charge, of the kraft white liquor is added to the wastewood as it enters the mixing eq ipment, prior to 4 entering the digester. By the time the wastewood reaches the digester, essentially every particle has ben wetted by the white liquor. The signals sent to the Chemical Proportioning Controller also calculated the need, if any, of additional white liquor, and water or black liquor needed for dilution in the digester to arrive at the desired liquorto-wastewood ratio. The amounts needed are controlled through valves 11 and 12, respectively.

In FIG. 2, a laboratory cook clearly shows one of the advantages of this invention in that using mixing, clean pulps can be prepared at a lower liquor-to-wastewood ratio than without mixing. This permits a large charge in a given digester and hence greater pulp production per cook. The liquor-to-wastewood ratio is defined as the total liquid volume contributed to the cook by the moisture in the wastewood, the white liquor and the dilution liquor. The dilution liquor can be water as in laboratory experiments or black liquor as in mill operations. As shown in FIG. 2 mixing may be obtained using a liquor (ml.) to wastewood (grams) ratio as low as 3 to 1 and as high as 7 to 1 (liquor to wastewood), a ratio of about 3.5-5 to 1 is preferred. If lower than 3 to l liquor to wastewood ratio is used, nonuniform cooking results, the reject rate increases, and the resultant pulp is contaminated with shives and uncooked Wood particles. Above about a 7 to 1 liquor to wastewood ratio is likewise harmful as dirty pulps are produced.

The thoroughly mixed wastewood and white liquor together with any dilution liquor is fed to a batch digester, the digester capped and the wastewood cooked in the following manner. In the first step, the digester is brought to a temperature of between 280 F. and 320 F., preferably about 300 F., to a pressure of 30 to 70 p.s.i.g., preferably about 50 p.s.i.g., and held for 20 to 50 minutes, preferably 30 minutes. This holding period, it is theorized, enables the cooking liquor to penetrate the wastewood par ticles and also permits air entrapped in the wastewood mass to escape.

In the second stage, the digester temperature is raised to between 330 F. and 360 F., preferably 345 F., at a pressure of between and 120 p.s.i.g., preferably p.s.i.g. The total elapsed cooking time is from about 90 to 180 minutes, preferably about minutes. The total elapsed cooking time includes time to first stage cooking temperature and cooking time during both stages.

The dirt level is determined by comparison of the oc currence of shives, bark and unfiberized Wood in a handsheet of the subject pulp with a series of standard handsheets having graduated dirt levels. The cleanest sheet contains approximately 2 objectionable dirt particles per 1.5 gram handsheet of pulp and is identified as a dirt level of 1. Each dirt level differs from the next lower dirt level by a factor of 2. A handsheet of dirt level 3 thus has twice as many dirt particles as one of dirt level 2 and 4 times as many as a sheet of dirt level 1.

The advantages of using the above-described process are numerous and include, among other; consistently clean, uniform pulps, a simple process not requiring high pressure mixing and thus expensive continuous process equipment, personnel training is quick and simple, and because of the moisture sensing arrangement any mixture of both kiln-dried and green material may be easily used. These advantages combine to provide a heretofore unknown uniquely flexible wastewood pulping process utilizing conventional batch digesters.

The practice of this invention may clearly be seen in the following examples.

EXAMPLE 1 From wastewood storage 35.5 wet tons [-17.9 (O.D*.) tons] of pine wastewood having a moisture content of 49.6% was conveyed to the Weightometer and an infrared absorption moisture gauge and the signals sent to the chemical proportioning controller. The signal from the chemical proportioning controller was to mix white liqnot at 15.9% effective alkali on O.D. wood at a liquorlto-wood ratio of 4.05:1 (mls./ gram). The wastewood and liquor were passed together through a mechanical mixing screw. Substantially all the wastewood particles were wetted upon leaving the screw.

The mixture was charged into a batch digester, 4,460 gallons of black liquor added and the digester capped. The mixture was then cooked in a first stage at 60 p.s.i.g. at 308 F. for 30 minutes. The temperature was then raised to 344 F. at 108 p.s.i.g. for another 45 minutes. The total cooking time including warm-up was 120 minutes.

The cook resulted in a Blow-Line K-No, 25.6, and an observed dirt level after double disc refining of 1.

EXAMPLE 2 The following cooks were made to illustrate various operating conditions. The mixed charge for the cooks were:

Wood charge Percent Liquor-to E.A. Wood ratio O.D. Percent on DD. "as charged Cook Number tons moisture wood mls/gram.

The two-stage cooking conditions were:

First stage cooking Second stage cooking Total Pres- Prescooksure, Temp., Time, sure, Temp., Time, ing, p.s.l.g. F. min. p.s.l.g. F. min. min.

Each of the cooks produced a clean pulp. The results of each cook are shown below.

While the invention has been described and illustrated herein by references to various specific materials, procedures and examples, it is understood that the invention is not restricted to the particular materials, combinations of materials, and procedures selected for that purpose. Numerous variations of such details can be employed, as will be appreciated by those skilled in the art.

What is claimed is:

1. The process for producing pulp from wastewood in a batch digester which comprises,

(a) prior to entering the digester mixing the wastewood throughly with Kraft cooking liquor,

(b) at a liquor to wastewood ratio of between 3:1 and 7:1, heating said liquor-wastewood mixture at a temperature from about 280 F. to 320 F. and pressure of from 30 to p.s.i.g. for from about 20 to 50 minutes, and

(c) digesting at a temperature from about 330 F. to 360 F. and pressure of from to p.s.i.g. for from 90 to minutes total cooking time.

2. The process according to claim 1 wherein the liquor to wastewood ratio is between 3.5 :1 and 5:1.

3. The process according to claim 1 wherein the moisture content and weight of the wood charged is used to automatically proportion the Kraft cooking liquor mixed with wastewood.

4. The process according to claim 3 wherein the addition of dilution liquor necessary to arrive at a predetermined liquor-to-wastewood ratio is automatically determined.

References Cited UNITED STATES PATENTS OTHER REFERENCES Canadian Pulp and Paper Association, Keller et al., Sulphite Pulps and Papers From Sawdust and Chip Mixtures, Pulp And Paper Magazine of Canada, June 1947, pp. 72-75.

S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner U.S. Cl. X.R. 162-71 

